It is neither common mode nor differential mode noise by itself. It is RF getting to the diodes on the input of your power supply which then modulate the RF. These are the diodes in the switching power supply that convert the incoming mains voltage to DC, prior to switching.

You should gain some insight from the article at the URL below.

http://home.computer.net/~pritch/shortwav.htm

Ah I see, I forgot there are still diodes in a SMPS to convert main AC to high voltage DC. Thanks DickCappels.

PS:
I did tried a few different SMPS, from cheap to decent one. They all have hum. Maybe I should try one from Apple, I was told they are well engineered. (If I can get an Apple phone/laptop charger)

 

It is called "Phantom Power". Condenser and electret microphone elements need a DC voltage to operate. The microphone input in a mixing console or other high end audio device has a DC voltage (usually +48 Vdc) impressed on the two signal lines through resistors (usually 6.8 K). There is not much current available, but it is enough to run a small preamp to boost the audio signal before its long journey through the cable to the console. Balanced audio techniaues eliminate a lot of induces crud, but nothing is better than having a loud, low output impedance signal out at the source.

https://en.wikipedia.org/wiki/Phantom_power

ak

Ah...I got it! when I first look at it, I was wondering where is the positive rail, I assumed that the top line is positive rail and the author just forgot to label it. Silly me

Thanks ak